Mexico City has treated its rivers badly: They tend to be paved over and filled with sewage:

Mexico City has treated its rivers badly: They tend to be paved over and filled with sewage. But Elías Cattan, a green building leader in Mexico City, wants to turn one of these f*cked-under resources back into a healthy, flowing river. Under Cattan’s guidance, the trash-clogged Río Piedad would become a viable waterway with a park on its banks and a transportation infrastructure dominated by walking, biking, and mass transit. Just give him two years and $1 billion. The city government does not share Cattan's enthusiasm and is more inclined to target less toxic and more easily mended rivers. (Río Piedad is pretty far gone -- its name even means “Pity River,” as if the garbage and concrete and feces weren’t pathos enough.) And $1 billion might be too low an estimate for the work it would take to clean the river of sewage. But as Mexico City Minister of the Environment Marth...